Because the assumption is that you are automatically unhealthy and over indulge right... Please do your research

Well yeah, actually by the general public that is the assumption, why should every person have to do research about that just to figure out why the main character of an anime is fat, instead the main character is healthy looking to the general public because that is more appealing and believable.

I hate to say this but the only group of people who would get behind a fat protag are fat people irl, and even then the only way they could write the character is like CHOJI from Naruto, more like fat satire than anything, and even Choji gets skinny.

Look if Naruto was fat so many little kids would think being fat is a positive thing, and it wouldn't be believable, I mean come on, a super fast ninja who weighs 350?

I'd be fine watching a series with an overweight or obese protagonist. Representation of the female form according to one's ideals is an artist's prerogative, but so too is representing the female form according to what is typical around the artist. Both are valid styles of representation, and provided the characters are well designed and experience development as the plot carries on I will be satisfied.

To be honest Ive never seen an anime where the protagonist is a fat girl that saves people and things like that.

To also be honest, perfect body female characters sell more than the same anime if you were to put overweight female characters in it. That is the reason they keep putting them in anime. Anime creators are in for the money, it makes financial sense to do it the way that sells best.

The protagonist could be an black overweight trans-gendered alien giraffe for all I care. What matters is what is done with the character. If the creator decides to break the tradition and make a fat protagonist, their weight will need to play some part in the story (otherwise, what's the point?). If that's the case, it is probably going to be boring as hell unless it takes a seriously dramatic turn.

Why are most progaonists fit? Because it suits their character. A lot of shows take the actiony approach, which means it wouldn't make a whole lot of sense to have an overweight protagonist that can perform acrobatic and awe-inspiring moves without making any progress. A slice of life could work, but like I said, it'll be boring unless there are some dramatic bits in there, which would probably end up sending the wrong message.

I think that a more serious twist on "It's not my fault I'm not popular" could work, as a 'study' on bullying, but eh.

And let's think of this illogically.
Anime are Japanese.
Japan does not consume nearly as much of the "fattening" foods we consume here.
Those who do, become hikikomori.
No hikikomori "saves people", or is that kind of protagonist.
If the protagonist was a fat girl, then she would already (hopefully) understand her appearance and that it has nothing to with what's inside.
Therefore, that girl has no reason to spend 11 episodes fretting over common love aspects that most people enjoy watching.

There are plenty of fat girl side characters, though. I think there's even an anime where the protagonist is a fat guy.
Plus, I think they might get lawsuits from "portraying the wrong message to young people."- or something like that.

If you want to see a fat anime girl protagonist so badly, why don't you make your own?

Technically, they could do it and make a killing if they dubbed it quick [America's a pretty big nation, no matter the reason. If ya know what I mean.] Plus, the main doesn't HAVE to be saving anything... it could just be a story about their struggles and how they do/don't take well to some of the main issues people are having today. Obesity, bullying, suicide, yadayadayada. They could survive to become a beautiful, happy woman... or commit suicide... whatever the outcome, it could end up a good story! ^_^

An comedy anime with a chubby girl would be fun something along the lines that Melissa McCarthy does, could even have some romance. I'm not picky if the story line is good then I'll watch almost anything.
Some people have said that anime is based on the average weight of Japanese girls, did you also know that teenage Japanese girls are famous for starving themselves? There was some research done not too long ago stating that most high school Japanese girls eat only 1 meal a day and was usually something small. So knowing this does that sound "healthy" to you? As for the haters on this thread, their ignorance makes me laugh.